hii readers,
Today, I m going to share with you actual method to calculate the checksum of TCP header packet at the binary level.
Introduction
Are you working on a networking project where you created TCP/UDP packets from scratch but don't know how to find checksum of from pseudo TCP packets? well, readers, you come to the right place. here, I am going to share with you the actual binary concept to calculate the checksum of pseudo TCP packets. Before starting this tutorial, I want to highlight that for this tutorial you need basic information of binary number operations.
How it works?
I am assuming that you already aware of TCP pseudo concept. basically, to calculate the checksum of TCP packet we need to create a TCP pseudo packet to calculate the actual checksum of TCP packet. if you don't know about pseudo concept then, I think you need to take a look
here.
Now, let's focus on theory again. first, we have to divide and slice binary data into 16 bits pieces. After slicing in 16-bit pieces, we have to do find out the binary addition of all sliced pieces. After getting the binary sum of 16-bit pieces, there is one more small trick before one's complement of the binary sum. actually, we just need to take a 4-bit number from starting point of binary number and then add it again with the main binary sum. At last, calculate one's complement of the binary sum. done! I hope now you got the basic concept of TCP Packet CheckSum.
Example To Understand Theory.
Step 1.Let assume that We have 4 bytes of pseudo header data (32-bits).
Binary Representation = 10000000111111100101011111001010
Step 2.Divide binary Number Into 16-bit pieces.
0 16-bit 32-bit
---------------------------------------
1000000011111110 0101011111001010
---------------------------------------
Step 3.Calculate the sum of all sliced pieces of binary numbers.
0-bit 16-bit
1000000011111110
+ 0101011111001010
----------------------
01101100011001000
-----------------------
Step 4.Add 4-bits of binary number into the calculated binary number. here, these 4 bits are nothing other than starting 4 digits of the same calculated binary number.
Calculated Binary Numbers = 01101100011001000
4-bit to add into same binary number = { 0110 } 1100011001000
Addtion:
0-bit 16-bit
01101100011001000
+ 0110
----------------------
01101100011001110
-----------------------
Step 5.Find One's Complement. (Just Change All 0's Into 1's).
Binary Number = 01101100011001110
One's Complement = 00010011100110001
Step 6. Done! That One's Complemented binary number is our checksum. hence, with this method, you can easily find the correct checksum of any TCP/UDP packets.
CheckSum = One's Complement = 00010011100110001
I hope you enjoyed this tutorial.
if you have any suggestion or query.
comment below
have a nice day.